Bitlocker & offline defrag apps

Thought i'd share this.
Sometimes the offline defrag facility in some 3rd party defrag apps doesn't
work on 3rd party encrypted volumes.
One reason for this is that the defrag app cannot lock the drive.

I had been told that Perfectdisk offline defrag couldn't defrag a Bitlocker
encrypted volume & that isn't true.
Perfectdisk offline defrag will defrag the C volume without problems, but it
cannot lock or perform a offline defrag of the boot S volume.

Online defrag works on both as normal.